Theoretical Study of the Photophysics of 8‑Vinylguanine, an Isomorphic Fluorescent Analogue of Guanine

Paving the way for the application of the algebraic-diagrammatic construction scheme of second-order (ADC(2)) to systems based on the guanine chromophore, we demonstrate the this excited-state electronic structure method provides a realistic description of the photochemistry of 9<i>H</i>-guanine, in close agreement with the benchmark provided by the CASPT2 method. We then proceed to apply the ADC(2) method to the photochemistry of 8-vinylguanine (8vG), a minimally modified analogue of guanine which, unlike the naturally occurring nucleobase, displays intense fluorescence, indicative of a much longer-lived excited electronic state. The emissive electronic state of 8vG is identified as an <i>ππ</i>*-type intramolecular charge transfer (ICT) state, in which a charge of roughly −0.2 <i>e</i> is transferred from the guanine moiety onto the vinyl substituent. The main radiationless deactivation pathway competing with fluorescence is predicted to involve the molecule leaving the minimum on the ICT <i>ππ</i>* state, and reaching a region of the S₁ adiabatic state where it resembles the L_a <i>ππ</i>* state of unmodified 9<i>H</i>-guanine. The topology of the L_a <i>ππ</i>* region of the S₁ state favors subsequent internal conversion at a crossing seam with the ground electronic state. The sensitivity of this process to environment polarity may explain the experimentally observed fluorescence quenching of 8vG upon incorporation in single- and double-stranded DNA